f2fs: introduce a periodic checkpoint flow

This patch introduces a periodic checkpoint feature.
Note that, this is not enforcing to conduct checkpoints very strictly in terms
of trigger timing, instead just hope to help user experiences.
The default value is 60 seconds.

Reviewed-by: Chao Yu <chao2.yu@samsung.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>

fs/f2fs/segment.c

@@ -15,6 +15,7 @@
 #include <linux/prefetch.h>
 #include <linux/kthread.h>
 #include <linux/swap.h>
+#include <linux/timer.h>
 
 #include "f2fs.h"
 #include "segment.h"
@@ -315,7 +316,8 @@ void f2fs_balance_fs_bg(struct f2fs_sb_info *sbi)
 	/* checkpoint is the only way to shrink partial cached entries */
 	if (!available_free_memory(sbi, NAT_ENTRIES) ||
 			excess_prefree_segs(sbi) ||
-			!available_free_memory(sbi, INO_ENTRIES))
+			!available_free_memory(sbi, INO_ENTRIES) ||
+			jiffies > sbi->cp_expires)
 		f2fs_sync_fs(sbi->sb, true);
 }